Cancer is a serious disease that accounts for a major cause of death. However, therapeutic needs therefor have not yet been met. In recent years, in order to overcome the problem of conventional chemotherapy that causes damage even to normal cells, studies have been intensively conducted regarding cancer therapy using molecularly targeted drugs, in which a drug targeting a specific molecule that is expressed specifically in a cancer cell is designed, and the therapy is then carried out using the drug.
As one of the targets, CDH3 (P-cadherin) which is a cell membrane surface antigen has been identified. CDH3 is a membrane protein that has been discovered as a molecule that is calcium-dependently associated with hemophilic cell adhesion (Yoshida and Takeichi, Cell 28: 217-224, 1982 (Non Patent Literature 1)). A protein, which has cadherin repeats consisting of approximately 110 amino acid residues having high homology to one another, is referred to as a “cadherin superfamily,” and CDH3 is a main member of the cadherin superfamily.
An increase in the expression of CDH3 in certain types of cancer cells has been reported. Thus, cancer therapy has been studied in which an antibody is used on cancer cells having higher expression of CDH3 in cancer tissues than in normal tissues (WO 2002/097395 (Patent Literature 1), WO 2007/102525 (Patent Literature 2), JP Patent Publication (Kohyo) No. 2011-526583 A (Patent Literature 4), and WO 2011/080796 A1 (Patent Literature 5)).
A large number of molecular-targeted drugs, which target a specific antigen as mentioned above, have already been placed on the market as antibody drugs, and a majority of the drugs have antibody-dependent cellular cytotoxicity (ADCC; Antibody Dependent Cellular Cytotoxicity) as a principal mode of action. However, their drug effects are not necessarily sufficient, and thus, technology development is proceeding towards the achievement of a stronger anticancer effect.
An effective means for enhancing the anticancer ability of an antibody is the binding of the antibody to a substance having strong toxicity (toxin). If toxin alone were administered to a patient, it would also affect normal tissues, and thereby, it could not be an effective therapeutic means. However, as a result of the binding of the toxin to an antibody that binds to a cancer cell-specific antigen, the toxin is able to achieve a capacity of killing only cancer cells, while it does not affect normal tissues. Such a drug is referred to as an antibody drug conjugate (ADC; Antibody Drug Conjugate). That is to say, a toxin shows no toxicity in a state in which it binds to an antibody. However, when a certain type of antibody binds to a cell that expresses a target antigen, it is incorporated into the cell and is then decomposed by a lysosome. Accordingly, the certain type of antibody, to which a toxin binds, is incorporated into the cell, and it is then decomposed therein, so that the toxin is released. As a result, the toxin is expressed only in a specific cell, and the cell is then killed by the effect thereof.
Examples of a drug ingredient used in ADC include bacterial protein toxins such as diphtheria toxin, vegetable protein toxins such as ricin, and low-molecular-weight toxins such as an auristatin, a maytansinoid or a calicheamicin and the derivatives thereof.
In ADC, a drug that is bound to an antibody circulates in the blood and then accumulates in a target tumor, and thereafter, it exhibits its drug effects. The release of a drug in sites other than tumor sites (the release from the antibody) is not necessarily preferable because it is likely to cause side effects. That is, a drug that is bound to an antibody is preferably designed such that it is removed from the antibody after it has been incorporated into a cell.
In recent years, from the aforementioned viewpoint, a drug (developed drug name: T-DM1), in which a toxin is bound, via a non-cleavable linker (SMCC), to trastuzumab that had already been commercially available as a therapeutic agent for breast cancer, has been developed by Genentech, and extremely high clinical effects have been obtained (N. Engl. J. Med. 2012 Nov 8; 367 (19): 1783-91 (Non Patent Literature 2)). In addition, an antibody drug conjugate, in which an antibody is bound to a drug component via a cleavable linker, has also been developed. For example, the development of an antibody drug conjugate, in which a drug is bound to a HuN901 antibody via a cleavable linker (SPP), that targets diseases expressing NCAM antigen, has been promoted by Immunogen.
Moreover, an agent for radioimmunotherapy, in which a radioactive material is bound to an antibody and the thus obtained antibody is subjected to the radioimmunotherapy, has also been developed. As a drug formed by binding a radioactive material 90Y (yttrium) or 111In (indium) to a chimeric anti-CD20 antibody, zevalin (common name: ibritumomab tiuxetan) has been placed on the market.
When the present antibody is used in the form of a drug conjugate or the like, and in particular, when the antibody is administered to a patient for a long period of time, the immunogenicity of the antibody to be administered that may generate an antibody against a heterologous immunoglobulin (e.g., a human anti-mouse antibody (HAMA)) is desirably a minimum or nothing. It is advantageous to produce a drug conjugate using such an antibody.
As a means for obtaining such an antibody, for example, a technique of producing a humanized antibody by combining the complementarity determining region (CDR) obtained from a heterologous organism such as a mouse with the framework region (FR) of an antibody derived from a human has been commonly used by a person skilled in the art (JP Patent Publication (Kokai) No. 2005-000169 A (Patent Literature 12), and Japanese Patent No. 4836147 (Patent Literature 13)). However, when inappropriate FR is combined with CDR, undesirable results such as the disappearance of affinity and a decrease in stability are frequently obtained. To cope with such phenomena, a method called “reshape,” which comprises substituting amino acid residues derived from the antibody serving as a transplant donor with amino acid residues in the corresponding positions in the framework region, has been carried out. If appropriate substitutions were carried out, a reduction in affinity possibly caused by humanization could be improved (Nature; 332, p. 323 (1988) (Non Patent Literature 3), and U. S. Patent No. 6180370 (Patent Literature 3)).
Hence, the CDR sequence derived from a heterologous organism such as a mouse and the FR sequence derived from a human, which are used in the present humanized antibody, are preferably 100% identical to their original amino acid sequences. However, substitution of amino acid residues is commonly carried out for the purpose of maintaining the binding of an antibody to an antigen in the process of humanization and chimerization. For the purpose of maintaining affinity, it is also preferable to add a genetic modification to an antibody within a range in which the binding ability of the antibody to CDH3 is maintained and the immunogenicity thereof is not extremely increased. The sequence consisting of CDR and FR, which are combined for humanization, shows a sequence homology of at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more, with the original sequence, or it is 100% identical to the original sequence. The antibody comprising such a partially modified sequence is considered to be an antibody that maintains the properties of CDR derived from the original hybridoma in the sense that it specifically binds to a specific epitope of CDH3.
Using the thus obtained humanized antibody, the immunogenicity of the antibody is kept to the minimum, and further, an immune complex comprising the humanized antibody, which has a strong cytotoxicity such as ADC, is provided for the treatment of diseases. This clearly benefits patients whom the drug is administered to. Moreover, in the present technical field, there is a further demand for drugs used to treat various cancers such as lung cancer, colon cancer and breast cancer. An example of such a drug that is particularly useful for this purpose is an anti-CDH3 humanized antibody drug conjugate, which has significantly low toxicity but has advantageous therapeutic effectiveness.